github-actions: comment cfg grammar diff in a separate workflow

As the check-cfg-grammar-changes workflow is triggered by the
pull_request event, it acquires the permissions from the originator repo
of the PR. If the originator repo is a fork, then it will not be able
to comment on an upstream PR.

The solution to this is introducing a workflow_run workflow, which gets
triggered by a completed check-cfg-grammar-changes run, runs in
the upstream repo, and has the necessary permissions to comment on a PR.

A workflow_run workflow always runs on the source code on master, so it
cannot be exploited by changing it in a PR.

Signed-off-by: Attila Szakacs <attila.szakacs@axoflow.com>

.github/workflows/check-cfg-grammar-changes.yml

@@ -1,8 +1,5 @@
 name: Check config grammar changes (PR)
 
-permissions:
-  pull-requests: write
-
 on:
   pull_request:
 
@@ -12,5 +9,5 @@ jobs:
     env:
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:
-      - uses: syslog-ng/syslog-ng-cfg-diff-pr@v1
+      - uses: syslog-ng/syslog-ng-cfg-diff-pr@v2
         continue-on-error: true

.github/workflows/comment-cfg-grammar-changes.yml

@@ -0,0 +1,92 @@
+name: Comment config grammar changes (PR)
+
+on:
+  workflow_run:
+    workflows: [Check config grammar changes (PR)]
+    types:
+      - completed
+
+permissions:
+  pull-requests: write
+
+jobs:
+  update-or-remove-comment-on-pr:
+    runs-on: ubuntu-latest
+    env:
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    steps:
+        # The download-artifact action cannot download artifacts from other workflows.
+        # Copied from https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_run
+      - name: Download comment artifact
+        uses: actions/github-script@v6
+        with:
+          script: |
+            let allArtifacts = await github.rest.actions.listWorkflowRunArtifacts({
+               owner: context.repo.owner,
+               repo: context.repo.repo,
+               run_id: context.payload.workflow_run.id,
+            });
+            let matchArtifact = allArtifacts.data.artifacts.filter((artifact) => {
+              return artifact.name == "comment"
+            })[0];
+            let download = await github.rest.actions.downloadArtifact({
+               owner: context.repo.owner,
+               repo: context.repo.repo,
+               artifact_id: matchArtifact.id,
+               archive_format: 'zip',
+            });
+            let fs = require('fs');
+            fs.writeFileSync(`${process.env.GITHUB_WORKSPACE}/comment.zip`, Buffer.from(download.data));
+
+      - name: Unzip comment artifact
+        run: unzip comment.zip
+
+      - name: Get PR ID
+        id: pr-id
+        run: |
+          # The pull_requests array always come empty when querying a workflow run's data if it is started from a fork. This might be a GitHub bug.
+          # There is also no json field in the `gh run view` command, which could give us the PR ID, so we can only query it based on the fork and branch.
+          # See https://github.com/orgs/community/discussions/25220
+
+          PR_ID=$(gh pr view -R "${{ github.repository }}" "${{ github.event.workflow_run.head_repository.owner.login }}:${{ github.event.workflow_run.head_branch }}" --json "number" --jq ".number")
+          echo "pr-id=${PR_ID}" >> $GITHUB_OUTPUT
+
+      - name: Update or remove PR comment
+        run: |
+          EXISTING_COMMENT_ID=$( \
+            gh api \
+              -H "Accept: application/vnd.github+json" \
+              -H "X-GitHub-Api-Version: 2022-11-28" \
+              /repos/${{ github.repository }}/issues/${{ steps.pr-id.outputs.pr-id }}/comments \
+              --jq '.[] | select(.user.login=="github-actions[bot]") | select(.user.type=="Bot") | select(.user.id==41898282) | select(.body | startswith("#### This Pull Request introduces config grammar changes")) | .id')
+
+          if [[ -s "${GITHUB_WORKSPACE}/comment" ]]; then
+            if [[ -n "${EXISTING_COMMENT_ID}" ]]; then
+              gh api \
+                -H "Accept: application/vnd.github+json" \
+                -H "X-GitHub-Api-Version: 2022-11-28" \
+                --method PATCH \
+                /repos/${{ github.repository }}/issues/comments/${EXISTING_COMMENT_ID} \
+                --input ${GITHUB_WORKSPACE}/comment
+              echo -e "\n::notice::Updated comment: ${EXISTING_COMMENT_ID}"
+            else
+              gh api \
+                -H "Accept: application/vnd.github+json" \
+                -H "X-GitHub-Api-Version: 2022-11-28" \
+                --method POST \
+                /repos/${{ github.repository }}/issues/${{ steps.pr-id.outputs.pr-id }}/comments \
+                --input ${GITHUB_WORKSPACE}/comment
+              echo -e "\n::notice::Created new comment"
+            fi
+          else
+            if [[ -n "${EXISTING_COMMENT_ID}" ]]; then
+              gh api \
+                -H "Accept: application/vnd.github+json" \
+                -H "X-GitHub-Api-Version: 2022-11-28" \
+                --method DELETE \
+                /repos/${{ github.repository }}/issues/comments/${EXISTING_COMMENT_ID}
+              echo -e "\n::notice::Removed comment: ${EXISTING_COMMENT_ID}"
+            else
+              echo -e "\n::notice::Nothing to do"
+            fi
+          fi